Electric railway vehicles such as locomotives or rail coaches powered by an alternating-current (AC) supply line use a traction transformer and an AC/DC converter for converting the high voltage (15 kV or 25 kV) of the supply line to a direct-current (DC) link voltage of a few kV and to ensure a galvanic isolation between the high voltage and the traction circuits. A DC link or bus at the DC link voltage feeds the drive or motor converters for traction or propulsion of the vehicle, as well as auxiliary converters for auxiliary energy supply. Hereby, the DC link voltage does not exceed the rating of the semiconductor devices in the motor or auxiliary converters. In modern railway vehicle concepts, the traction transformer is usually placed outside the main casing of the vehicle, i.e. under floor or on the rooftop. Under these circumstances however, a conventional transformer with a nominal frequency of 16.7 Hz or 50 Hz causes integration problems due to its high weight and large volume.
Alternative power supply systems therefore aim at replacing the aforementioned conventional transformer by additional converters based on semiconductor technology in combination with a smaller and lighter transformer operating at a higher frequency. At the expense of switching losses in the semiconductor devices, the mass and volume of the transformer as well as the total, i.e. copper and magnetic, losses in the transformer can thus be reduced, resulting in a more efficient use of the electrical power from the supply line.
In the patent application EP-A 1 226 994, a medium frequency power supply system for rail vehicles is presented, including a classical converter topology for the bidirectional conversion of a high input AC voltage to a DC output voltage. The system comprises a primary converter composed of at least three cascaded converter modules or sections electrically connected in series, one single common transformer and a single secondary converter. Each cascade module in turn is formed by a four-quadrant converter, a 3.6 kV DC intermediate stage and a resonant converter. The latter comprises a half bridge with two circuit elements each having a transistor and a free-wheeling diode, as well as a series resonance capacitor. Together with the corresponding primary leakage inductance coil of the transformer, the series resonance capacitor forms an oscillating circuit having a resonance frequency higher than the transformer nominal frequency.
The secondary or output converter is a resonant switched four-quadrant converter feeding the vehicle's 1.65 kV DC link. All switching elements are advanced 6.5 kV Insulated Gate Bipolar Transistors (IGBT) with an adapted gate driver technology. The medium-frequency (MF) transformer has a ferrite core and a nominal frequency of 5 kHz, which is considered to be the optimum compromise between the volume and weight of the transformer and the switching losses of the semiconductor switches. The provision of only one transformer and only one secondary converter is expected to save not only costs but also weight, but obviously impairs the availability of the power supply system.
Instead of passing through a DC intermediate energy storage stage, conversion from one frequency to another can be achieved directly from an AC input to an AC output by a direct AC frequency converter, also known as a cycloconverter. By way of example, DE 2614445 discloses a rectifier converting low frequency AC voltage into DC voltage via a transformer operating at medium/high frequencies that comprises, on the AC side of the transformer, an externally controlled, single-phase bridge cycloconverter. The latter comprises a single conversion stage between the low frequency of the supply line and the medium/high frequency of the transformer, and thus presents fewer power conversion stages and no crowbar circuit as compared to the abovementioned cascade modules including an intermediate DC stage. Furthermore, the cycloconverter operates with natural commutations. On the other hand, a voltage source converter (VSC) on the DC side of the transformer operates with hard switching off, resulting in comparatively high stresses on the semiconductor devices of the current valves and high switching losses.
In the article by Staffan Norrga, “An Experimental Study of a Soft-switched Isolated AC/DC Converter without Auxiliary Circuit”, 35th Annual IEEE Power Electronics Specialists Conference, Aachen, Germany, 2004, pages 2406-2412, a single-level, single-phase AC/DC converter for bidirectional power flow is presented. The converter topology incorporates a single-phase cycloconverter coupled by a medium frequency (MF) transformer to a voltage source converter (VSC). The VSC has so-called snubber capacitors connected in parallel to each of the semiconductor valves, providing a sufficient capacity to allow for zero-voltage turn-off of said valves and reducing the voltage derivatives when the valves are switched. On the other hand, the VSC has no additional or auxiliary semiconductor devices that do not take part in the power conversion itself and merely add to the costs and complexity of the converter. Following the commutation algorithm proposed, this topology allows to obtain soft switching of the semiconductor valves in the VSC-converter in all points of operation. A prototype converter system has been realised with a DC link voltage of 600 V and a ferrite core toroidal MF transformer operating at 6 kHz. The bidirectional valves of the cycloconverter are made up of two single, individually controlled, standard 1200 V IGBT switches in common-emitter connection.
The article “High-Frequency Link DC/AC Converter with Suppressed Voltage Clamp Circuits—Naturally Commutated Phase Angle Control with Self Turn-Off Devices” by Mikihiko Matsui et al., IEEE Trans. Ind. Appl., Vol. 32, No. 2, 1996, pages 293-300, proposes an alternative control for a cycloconverter-type, three-phase output, high frequency (HF) link DC/AC converter for e.g. uninterruptible power supply (UPS) systems. The converter has two power conversion stages, i.e. the inverter stage for DC to HF (16 kHz) and the cycloconverter stage for HF to the low or commercial frequency of the AC power line. Because of the use of self turn-off devices (bidirectional transistors and fast recovery diodes), the proposed control approach requires modified control pulse timing and adopts only phase angle control based on natural commutation for the cycloconverter stage. As the magnetic energy stored in the leakage inductances of the HF transformer at the moment of switching is released and feedback to the DC voltage source, there is no need for additional surge suppressing circuits (snubber circuit or active voltage lamp circuit) on the cycloconverter side of the transformer for absorbing said magnetic energy.